1. Field of the Invention
The present invention is directed to a structure for a migration imaging member for which uniform charging is more reliably achieved.
2. Description of Related Art
Heat developable migration imaging members are typically very thin, which allows charging to high charge density and high field strength with relatively low surface potentials. This creates problems for uniform charging because the ratio of surface potential to the electrostatic fields associated with the surface charges is too low.
Migration imaging members are well known, and are described in detail in, for example, U.S. Pat, No. 3,975,195 (Goffe), U.S. Pat. No. 3,909,262 (Goffe) et al., U.S. Pat. No. 4,536,457 (Tam), U.S. Pat. No. 4,536,458 (Ng), U.S. Pat. No. 4,013,462 (Goffe et al.), and "Migration Imaging Mechanisms, Exploitation, and Future Prospects of Unique Photographic Technologies, XDM and AMEN", P. S. Vincett, G. J. Kovacs, M. C. Tam, A. L. Pundsack, and P. H. Soden, Journal of Imaging Science 30 (4) July/August, pp. 183-191 (1986), the disclosures of each of which are totally incorporated herein by reference. Migration imaging members containing charge transport materials in the softenable layer are known, and disclosed, for example in U.S. Pat. No. 4,535,457 (Tam) and U.S. Pat. No. 4,536,458 (Ng), the disclosures of each of which are totally incorporated herein by reference. A typical migration member comprises a substrate, a layer of softenable material, and photosensitive marking material in the form of a fracturable layer contiguous with the upper surface of the softenable layer. The member is imaged by first electrically charging the member and exposing the charged member to a pattern of activation electromagnetic radiation, such as light, to form a latent image on the member. Subsequently, the imaged member is developed by one of several methods, such as application of heat, solvent, solvent vapor, or the like, causing the marking material in the exposed areas of the member to migrate in depth through the softenable material toward the substrate.
The expression "softenable" as used herein is intended to mean any material which can be rendered more permeable, thereby enabling the particles to migrate through its bulk. Conventionally, changing the permeability of such material or reducing its resistance to migration of migration marking material is accomplished by dissolving, swelling, melting, or softening, by techniques, for example, such as contacting with heat, vapors, partial solvents, solvent vapors, solvents, and combinations thereof, or by otherwise reducing the viscosity of the softenable material by any suitable means.
Typically, conventional scorotrons, for example, 4,591,713, are used to charge a photoreceptor to about 1000 V+/-3% or about 30 V. VerdeFilm (such as disclosed in U.S. Pat. No. 5,411,825 (Tam) and totally included herein by reference), in contrast, has to be charged to an even higher charge density, and with the same percentage of uniformity. A problem lies in the fact that the conventional VerdeFilm is only about 1 micron thick, and must be charged to a surface potential of only about 100 V. The 30 volt variation that is typical of scorotron charging represents only 3% for a photoreceptor that holds a surface potential of 1000 volts, but for a very thin photoreceptor that can support only 100 volts, the 30 volt variation now becomes 30%, which is not acceptable. Even the absolute value of the variation tends to increase for lower voltage charging requirements. For example, when charging to 1000 V the scorotron screen potential is dominant, but when the screen voltage is reduced by an order of magnitude, the relative coronode and the space charge contributions to the asymptote surface potential grow by an order of magnitude.